ifdef and ifndef Directives

ifdef directive

Content between the ifdef and endif directives gets included if the specified attribute is set:

Example 1. ifdef example

ifdef::env-github[]
This content is for GitHub only.
endif::[]

The syntax of the start directive is ifdef::<attribute>[], where <attribute> is the name of an attribute.

Keep in mind that the content is not limited to a single line. You can have any amount of content between the ifdef and endif directives.

If you have a large amount of content inside the ifdef directive, you may find it more readable to use the long-form version of the directive, in which the attribute (aka condition) is referenced again in the endif directive.

Example 2. ifdef long-form example

ifdef::env-github[]
This content is for GitHub only.

So much content in this section, I'd get confused reading the source without the closing `ifdef` directive.

It isn't necessary for short blocks, but if you are conditionally including a section it may be something worth considering.

Other readers reviewing your docs source code may go cross-eyed when reading your source docs if you don't.
endif::env-github[]

If you’re only dealing with a single line of text, you can put the content directly inside the square brackets and drop the endif directive.

Example 3. ifdef single line example

ifdef::revnumber[This document has a version number of {revnumber}.]

The single-line block above is equivalent to this formal ifdef directive:

ifdef::revnumber[]
This document has a version number of {revnumber}.
endif::[]

ifndef directive

ifndef is the logical opposite of ifdef. Content between ifndef and endif gets included only if the specified attribute is not set:

Example 4. ifndef example

ifndef::env-github[]
This content is not shown on GitHub.
endif::[]

The syntax of the start directive is ifndef::<attribute>[], where <attribute> is the name of an attribute.

The ifndef directive supports the same single-line and long-form variants as ifdef.

Checking multiple attributes

Both the ifdef and ifndef directives accept multiple attribute names. The combinator can be “and” or “or”.

Any attribute (or)

Multiple comma-separated (,) directive names evaluate to true, allowing the content to be included, if one or more of the directives is defined. Otherwise the content is not included.

Example 5. Any attribute example

ifdef::backend-html5,backend-docbook5[Only shown when converting to HTML5 or DocBook 5.]

All attributes (and)

Multiple plus-separated (+) directive names evaluate to true, allowing the content to be included, if all of the directives are defined. Otherwise the content is not included.

Example 6. All attributes example

ifdef::env-github+backend-html5[Only shown when converting to HTML5 on GitHub.]

The ifndef directive negates the results of the expression.